This paper provides single and two phase rod bundle data to support verification of heat transfer models being used in steaming rate and crud model predictions for rod bundles. The effort to summarize this work was supported by the EPRI Robust Fuel Program and is defined in more detail in EPRI report 1003383. Subcooled boiling tests were performed by Combustion Engineering (CE) in early 1980s to provide insight on heavy crud deposits and fuel failures observed on peripheral rods for bundles in Maine Yankee cycle 4. Two 5×5 tests were performed at the Columbia University Heat Transfer Research Facility simulating the peripheral region of adjacent CE 14×14 fuel bundles for two different perimeter strip geometries. The test conditions were at typical reactor pressure, temperature, and heat flux. The rods were 7’ in heated length and were electrically heated with a uniform axial power shape. There were no mixing vanes on the spacer grids. Thermocouples were placed on the hot rod in the center of the test section and on an adjacent rod at 4 different axial levels. Thermocouples were also located in the center of the subchannels at the end of the test section. Boiling curves were generated over a range of test conditions (system pressure, inlet temperature, and flow rate) by plotting rod surface temperature versus heat flux. The boiling curves covered single phase, subcooled boiling, bulk boiling and DNB conditions. The data from the boiling curves were reduced and evaluated with the VIPRE thermal hydraulic code. Clad temperature predictions were made with VIPRE based on available heat transfer correlations for comparison to clad temperature measurements. These heat transfer correlations include the Dittus Boelter correlation for single phase flow, the Jens Lottes, Thom and Chen correlations for two phase flow conditions (subcooled boiling). The VIPRE predictions of the hot rod average surface temperature, based on the Dittus-Boelter correlation with a grid enhancement factor for single-phase forced convection and the Thom correlation for nucleate boiling, gave the best agreement with the rod bundle test data among all the available modeling options. It was concluded that current heat transfer models used in TH codes, are adequate for average steaming rate calculations supporting Axial Offset Anomalies (AOA) evaluations, as long as the appropriate grid enhancement factor is utilized for the spacer grids in the analysis. However, further testing and modeling may be needed to simulate local grid effects and hot spots downstream of spacer grids.
Single-phase convective heat transfer enhancement by spacer grids in a rod bundle
